1. Field of the Invention
The present invention relates to an apparatus for high-efficiency coding for compressing a digital video signal at a high efficiency, and more particularly to an apparatus for high-efficiency coding of a digital video signal which controls the coding bit rate of the compressed digital data so as to keep it constant.
2. Description of the Prior Art
As an apparatus for the high-efficiency coding of a digital video signal, the Japanese Laid-open Patent No. 61-144989 discloses an apparatus for determining the dynamic range defined by the maximum and minimum values of plural pixels contained in a two-dimensional block, and coding at a variable bit length corresponding to this dynamic range. Japanese Laid-open Patent No. 63-111781 discloses a constant rate instead of a variable length coding, and an apparatus for high-efficiency coding was proposed for detecting the occurrence frequency of dynamic ranges in a specific period, sequentially integrating the frequency of the adjacent dynamic ranges by starting from the maximum value or minimum value to form an integration table of the occurrence frequency, and controlling the quantity of coded digital data using the integration table.
In the composition of controlling the quantity of coded digital data by using the integration table of the occurrence frequency as mentioned above, however, the number of quantization bits assigned for each dynamic range will merely increase monotonously with an increase of the dynamic range. In this simple method of assigning the number of quantization bits, when the dynamic range becomes larger, the distortion occurring between the signal decoded from the coded digital data and the original signal (hereinafter called distortion by coding) becomes extremely large at certain dynamic ranges. On the other hand, as the characteristics of human visual sensation, the characteristics of limit of impairment perception were reported, using the level deviation within a minute subpicture block of an image, or the dynamic range, as the parameter (see the 5th Picture Coding Symposium of Japan; A study on high-efficiency coding of HDTV signal, Shibata et al., pp. 147-148), and the above phenomenon does not match with human visual sensation, and may cause a loss in the picture quality.